On Saturday, Trump finally signed off on threatened 25-percent tariffs on neighboring Mexico and Canada -- despite sharing a free trade pact -- and hit China with a 10-percent tariff in addition to already enacted levies.

The president had vowed since before his inauguration to take such action, claiming the countries were not doing enough to halt illegal immigration and the trafficking of the deadly opioid fentanyl into the United States.

In imposing the tariffs, which are set to begin Tuesday, Trump invoked the International Emergency Economic Powers Act.

The move provoked immediate vows of retaliation from all three countries, while analysts warned that the ensuing trade war would likely slow US growth and raise consumer prices over the short term.

"Will there be some pain? Yes, maybe (and maybe not!)" Trump wrote Sunday morning in all-caps on his Truth Social media platform.

"But we will Make America Great Again, and it will all be worth the price that must be paid."

The president and his advisors had previously resisted acknowledging that tariffs could raise US consumer prices, after frustration over rising costs was seen as a major factor in his November election victory over Democrat Kamala Harris.

Apparently seeking to limit a spike in fuel and electricity prices, Trump put the levy on energy imports from Canada at only 10 percent.

In a separate social media post, Trump called again for America's northern neighbor to become a US state, heightening tensions further with one of his country's closest allies.

While claiming the United States pays "hundreds of billions of dollars to SUBSIDIZE Canada," Trump added, "Without this massive subsidy, Canada ceases to exist as a viable Country."

"Therefore, Canada should become our Cherished 51st State," he wrote on Truth Social, claiming the move would bring "much lower taxes, and far better military protection for the people of Canada - AND NO TARIFFS!"

The US Census Bureau listed the country's 2024 trade deficit in goods with Canada as $55 billion.

- 'Ripoff of America' -

Canadian Prime Minister Justin Trudeau vowed Saturday that his country would hit back with 25 percent levies of its own on select American goods worth Can$155 billion (US$106.6 billion), with a first round on Tuesday followed by a second one in three weeks.

Leaders of several Canadian provinces have already announced retaliatory actions as well, such as the immediate halt of US liquor purchases.

Mexican President Claudia Sheinbaum meanwhile said she had directed her economy minister to "implement Plan B," which includes yet-unspecified "tariff and non-tariff measures."

On Friday, the right-leaning editorial board of the Wall Street Journal newspaper blasted Trump's tariffs in a piece titled "The Dumbest Trade War in History," saying, "American consumers will feel the bite of higher costs for some goods."

Trump clapped back on Sunday, saying: "The 'Tariff Lobby,' headed by the Globalist, and always wrong, Wall Street Journal, is working hard to justify... the decades long RIPOFF OF AMERICA, both with regard to TRADE, CRIME, AND POISONOUS DRUGS."

He has long decried US trade deficits as a sign of other countries taking advantage of Americans.

"THOSE DAYS ARE OVER!" said Trump, who began his Sunday with a visit to one of his golf courses in Florida.

He has also repeatedly threatened trade actions against the European Union. A spokesperson for the bloc vowed Sunday that it would "respond firmly to any trading partner that unfairly or arbitrarily imposes tariffs."